The present invention relates to a method of and an apparatus for optical information reproduction in which a light spot traces an information track consisting of a sequence of pits by which information is recorded on an information recording medium, to read the recorded information. More particularly, the present invention relates to tracking control for guiding the light beam converged onto the medium, along the center of the track.
The invention also relates to an offset removing circuit used in such an optical information reproduction apparatus.
In recent years, optical disks such as DVDs which can be used for optical recording and reproduction of information are drawing attention as information medium capable of storing a large amount of video information and computer data. The optical disks are provided with concentric or spiral tracks at a pitch of about 1 xcexcm, and the information is recorded along the track by means of variations in a local optical constant or physical shape.
In order to reproduce information from the optical disk of this form of recording with a high quality, the optical information reproducing apparatus controls the position of convergence of the light spot for reading the information, so that the light spot keeps tracing the track. The position control of the light spot is effected in two dimensions. The control in the direction of the optical axis is effected by a focus control means, while the control in the radial direction of the disk is effected by a tracking control means. These controls are effected by feedback control in which the position of the light spot is controlled so as to eliminate the error which is the difference between the target position of the light spot and the current position.
Various methods have been devised for producing the tracking error signal necessary for the tracking control by an optical means. Among these various methods is a phase difference method which uses a signal obtained from a main light spot for reproducing the information recorded on the information medium. The principle of the phase difference method is disclosed in Japanese Patent Kokal Publication No. 52-93222, and its counterpart, U.S. Pat. No. 4,057,833 to Braat.
FIG. 5A to FIG. 5E are drawings for explaining the principle of detection of the tracking error information in the phase difference method. FIG. 5A shows the relative positions of the information pits and the light spot. It shows how the light spot moves in the direction of from time t0 to t4. The running position, point (xcex2), of the light spot is the center of the track from which information is to be reproduced. The point (xcex1) is on the left side of the center of the track from which information is to be reproduced. The point (xcex3) is on the right side of the center of the track from which information is to be reproduced.
FIG. 5B shows the photo-electric conversion means which detects the light reflected from the information medium, and converts it into electrical signals. The illustrated photoelectric conversion means is divided into first to fourth optical detectors by a division line extending in the direction corresponding to a track tangential direction, and a division line in the direction corresponding to a direction perpendicular to the track tangential direction. Ideally, the optical system is so designed that the center of the far-field pattern of the light reflected from the recording medium is formed at the center of the four optical detectors.
In other words, the first to fourth optical detectors are situated in the far-field of the information pits in separate quadrants of an imaginary X-Y coordinate system, whose origin is disposed on an optical axis of the optical system and whose X-axis effectively extends in the track tangential direction TT and whose Y-axis effectively extends transversely to the track tangential direction TT.
The first and second optical detectors are disposed on one side of the Y-axis. The third optical detector is disposed on the other side of the Y-axis, and disposed diagonally with respect to the first optical detector. The fourth optical detector is disposed on the other side of the Y-axis, and disposed diagonally with respect to the second optical detector.
The phrase xe2x80x9cthe optical detectors are situated in the far field of the information pitsxe2x80x9d is to be understood to mean that these detectors are located in a plane in which the different orders of the light beam reflected from the information medium are sufficiently distinct, i.e., in a plane which is sufficiently far from the image of the information pits.
The phrase xe2x80x9cthe X-axis effectively extends in the track tangential direction and the Y-axis effectively extends transversely to the track tangential direction,xe2x80x9d is to be understood to mean that the imaginary projections of these axes on the information pits extend in the track tangential direction, and transversely to the track tangential direction.
Further explanation is given in U.S. Pat. No. 4,057,833, which is hereby incorporated by reference.
A phase difference is present between the two detection signals (A+C) and (B+D) obtained by adding the outputs of the optical detectors disposed diagonally to each other, and the phase difference is proportional to the off-track amount, i.e., the amount by which the light spot is deviated from the center of the track. This is shown in FIG. 5C, FIG. 5D and FIG. 5E.
FIG. 5C shows how the phase relationship between the two detection signals varies with the scanning position of the light spot. The waveforms on the left are the detection signal waveforms obtained when the light spot scans a point (xcex1), i.e., on the left side of the center of the pit. As will be seen, the detection signal (A+C) is leading the detection signal (B+D) in phase. The waveforms in the center are the detection signal waveforms obtained when the light spot scans a point (xcex2), i.e., the center of the pit. As will be seen, the detection signal (A+C) and the detection signal (B+D) are in phase. The waveforms on the right are the detection signal waveforms obtained when the light spot scans a point (xcex3), i.e., on the right side of the center of the pit. As will be seen, the detection signal (A+C) is lagging behind the detection signal (B+D) in phase.
FIG. 5D shows the phase difference between the detection signal (A+C) and the detection signal (B+D) with respect to the scanning position of the light spot. The phase difference is represented by the pulse width. The pulse on the xe2x80x9c+xe2x80x9d side (above 0 level) indicates that the detection signal (A+C) is leading the detection signal (B+D), while the pulse on the xe2x80x9cxe2x88x92xe2x80x9d side (below 0 level) indicates that the detection signal (A+C) is lagging behind the detection signal (B+D). When the detection signal (A+C) and the detection signal (B+D) are in phase, no pulse occurs in the xe2x80x9c+xe2x80x9d side and the xe2x80x9cxe2x88x92xe2x80x9d side.
FIG. 5E shows the pulse width, i.e., the phase difference with respect to the scanning position of the light spot, and how it is in proportion with the off-track amount from the track center. This phase difference is converted into electrical signals, and a tracking error signal necessary for tracking is thus obtained.
It is known that the tracking error signal is associated with an offset (hereinafter referred to as xe2x80x9cfirst offsetxe2x80x9d) which is dependent on the pit depth. The details is described on pp. 33-38 of Technical Paper of the Institute of Electronics and Communication Engineers of Japan, OPE 96-150, xe2x80x9cDevelopment of a High-Precision Learning Control Method in a DVD-ROM Drive.xe2x80x9d
FIG. 6A to FIG. 6D illustrates the principle of the offset generation. The drawing shows the waveforms of the outputs (A to D) of the four detectors obtained when the light beam is positioned at the center of the track. The pit depth and the presence or absence of the lens shift are taken as parameters.
When the pit depth is xcex/4 (xcex being the wavelength of light from a laser diode (hereinafter referred to as LD)), the waveform patterns of the (A+C) signal and the (B+D) signal obtained from the optical detectors in the respective quadrants of the X-Y coordinate system explained above are identical, and even if the lens is shifted and the light spot on the optical detectors moves, the phase difference between the (A+C) signal and the (B+D) signal is zero as long as the light spot is on the center of the track.
When the pit depth is other than xcex/4, a level difference is present between the (A+C) signal and the (B+D) signal. When the reflected light on the optical detectors does not move, there is no level difference between the (A+C) signal and the (B+D) signal, and the tracking error signal is zero. When the lens moves, an imbalance is generated between the (A+C) signal and the (B+D) signal. As a result, a phase difference is generated and the first offset is generated in the tracking error signal.
A conventional optical information reproducing apparatus for obtaining a tracking error signal using the phase difference method having the above-described characteristics will next be described with reference to FIG. 7. In FIG. 7, reference numeral 1 denotes an information medium, 2 denotes an optical head, 3 denotes a first phase adjusting means, 4 denotes a second phase adjusting means, 5 denotes a phase adjustment amount setting means, 6 denotes a phase difference detecting means, 7 denotes an offset correction learning means, 8 denotes a tracking control means, 9 denotes a first switch means, and 10 denotes a driver. The optical head 2 comprises an LD 21, a beam splitter (hereinafter abbreviated as BS) 22, an actuator 23, a lens 24, and a photo-electric conversion means 25. The phase difference detection means 6 comprises a first addition means 61, a second addition means 62, a first comparator 63, a second comparator 64, a phase comparison means 65, and a phase difference-to-voltage conversion means 66. The offset correction leaning means 7 comprises a waveform symmetry measuring means 71 and a controller 72.
The operation of the conventional optical information reproducing means configured as described above will next be described with reference to FIG. 7. The optical output from the LD 21 forming the optical head 2 is controlled by a laser power control means, not shown, so that the light as incident on the information medium 1 is of a predetermined power. The optical beam emitted from the LD 21 is converted to parallel light by a collimator means, not shown, and is incident on the BS 22. The BS 22 has such characteristics that the light incident from the LD 21 is passed, while the light from the information medium 1 is reflected. The light having passed the BS 22 is converged onto a center of an information track on the information medium 1, by a lens 24 controlled by the actuator 23.
The light reflected from the information medium 1 is passed through the lens 24, and is reflected at the BS 22, and is incident on the photo-electric conversion means 25. The photo-electric conversion means 25 is divided into first to fourth optical detectors 25a to 25d by a division line extending in the track tangential direction TT, and a division line extending in the radial direction, which is perpendicular to the tangential direction, detects the reflected light from the information medium, which contains information of pits formed on the information medium 1, and converts the reflected light into electrical signals.
The division is so designed that the center of the far-field pattern is formed at the center of the photo-electric conversion means 25 in an ideal state in which the light spot is tracing the center of the track having pits with a depth of xcex/4. The positional relationship between the first to fourth optical detectors 25a to 25d is such that the first and second optical detectors 25a and 25b are disposed on one side of the division line extending in the perpendicular direction, and the third and fourth optical detectors 25c and 25d are on the other side of the division line extending in the perpendicular direction. The first and third optical detectors 25a and 25c are disposed at diagonal positions, and the second and fourth optical detectors 25b and 25d are disposed at the other diagonal positions.
In other words, the first to fourth optical detectors 25a-25d are situated in the far-field of the information pits in separate quadrants of an imaginary X-Y coordinate system, whose origin is disposed on an optical axis of the optical system (comprising the BS 22 and the lens 24) and whose X-axis effectively extends in the track tangential direction TT and whose Y-axis effectively extends transversely to the track tangential direction TT.
The first and second optical detectors 25a and 25b are disposed on one side of the Y-axis. The third optical detector 25c is disposed on the other side of the Y-axis, and disposed diagonally with respect to the first optical detector 25a. The fourth optical detector 25d is disposed on the other side of the Y-axis, and disposed diagonally with respect to the second optical detector 25b. 
The optical system is so designed that a phase difference proportional to the offset amount is present between the two detection signals (A+C) and (B+D) obtained by adding the diagonal components of the outputs A, B, C and D of the first to fourth optical detectors.
When the pit depth is other than xcex/4, a phase difference is generated between the (A+C) signal and the (B+D) signal due to imbalance, and this forms a first offset in the tracking error signal. In order to cancel the first offset, the conventional optical information reproducing apparatus adjusts the phase of the output A of the first optical detector and the output B of the second optical detector, by means of the first phase adjustment means 3 and the second phase adjustment means 4, so as to adjust the phase relationship with respect to the output C of the third optical detector and the output D of the fourth optical detector.
The optimum value of the phase adjustment amount depends on the depth of the pit. The phase adjustment amounts set by the first phase adjustment means 3 and the second phase adjustment means 4 are controlled to such values that the offset correction learning means 7 and the phase adjustment amount setting means 5 yield the best tracking error signal.
The phase difference detection means 6 detects the tracking error signal from the output Axe2x80x2 of the first phase adjustment means 3, the output Bxe2x80x2 of the second phase adjustment means 4, the output C of the third optical detector, and the output D of the fourth optical detector D, and through the following process.
The output Axe2x80x2 of the first phase adjusting means 3 and the output C of the third optical detector disposed at a diagonal position of the first optical detector are added at the first adding means 61 which is a component of the phase difference detecting means 6, and then binarized at the first comparator 63. The output Bxe2x80x2 of the second phase adjusting means 4 and the output D of the fourth optical detector disposed at a diagonal position of the second optical detector are added at the second adding means 62, and then binarized at the second comparator 64.
The phase difference between the two binary signals from the first comparator 63 and the second comparator 64 is detected at the phase comparison means 65, and is converted at the phase difference-to-voltage conversion means 66, to produce the phase difference tracking error signal. In the prior art, a low-pass filter (hereinafter referred to as LPF) is used as the phase difference-to-voltage conversion means.
The tracking error signal thus detected is input to the offset correction learning means 7 and the tracking control means 8. The offset correction learning means 7 measures the symmetry of the tracking error signal, by means of the waveform symmetry measuring means 71. The controller 72 controls the phase adjustment amounts of the first phase adjustment means 3 and the second adjustment amount means 4, via the phase adjustment amount setting means 5, so as to maximize the symmetry. The offset correction learning algorithm followed is shown in FIG. 8.
When the operation in the offset correction learning mode is started (S1), the controller 72 controls the switching means 9 so that the output of the controller 72 is input to the driver 10. The tracking control is thereby disabled, and the lens 24 is driven radially inward under control by the controller 72 (S2). In this state, the controller 72 controls, via the phase adjustment amount setting means 5, the first phase adjusting means 3 and the second phase adjusting means 4, and determines the phase adjustment amount which maximizes the symmetry of the tracking error signal (S3).
Next, the controller 72 drives the lens 24 radially outward (S4). In this state, the controller 72 controls, via the phase adjustment amount setting means 5, the first phase adjusting means 3, and the second phase adjusting means 4, and determines the phase adjustment amount which maximizes the symmetry of the tracking error signal (S5).
Lastly, the controller 72 determines the phase adjustment amount which minimizes the difference between the symmetries of the tracking error signals, based on the best phase adjustment amounts on the radially inner and radially outer sides determined at the steps S3 and S5, and sets the value in the first phase adjusting means 3 and the second phase adjusting means 4 (S6).
When the operation in the offset correction learning mode is completed, the controller 72 then switches the first switch 9 so that the output of the tracking control means 8 is input to the driver 10. The tracking control means 8 controls, via the drier 10 and the actuator 23, the lens 24 in the radial direction, so as to eliminate the tracking error detected by the phase difference detecting means 6, and that the light beam emitted from the light head on the information medium traces the center of the track.
The conventional optical information reproducing apparatus uses the symmetry of the waveform of the tracking error signal, i.e., the amount of shift of the center level of the reproduced waveform from the reference level, as information for identifying the best phase adjustment amount by means of the phase adjusting means. To measure the symmetry of the reproduced waveform, it is necessary to determine the local maximum and local minimum of the tracking error signal. As a means for determining the local maximum and local minimum, a measuring means which digitally process the signal obtained by discretely sampling the signal level by means of an analog-to-digital converting circuit (hereinafter referred to as ADC), and a measuring means which detects, in an analog manner, the envelope of the peak and the envelope of the bottom of the tracking error signal, and determines the median point thereof. In the case of the measuring means using the ADC, points which are not at the local maximum and local minimum (which are instantaneous values), but which are near them may be sampled, depending on the sampling rate. In such a situation, measuring errors are contained in the median point between the local maximum and the local minimum measured, and the quality of the tracking error signal will be deteriorated. If it is attempted to reduce the measuring errors by the use of a high-speed ADC, the cost of the apparatus may be increased. If it is attempted to reduce the measurement errors by the use of analog means, two series of envelope measurement circuits are required, increasing the size of the circuit.
In addition, the conventional optical information reproducing apparatus cannot cancel the electrical offset occurring in the circuits after the phase comparison circuit. This means that whether the offset in the tracking error signal is due to the electrical offset or the first offset caused by the improper setting of the phase adjusting means is not known, and the offset correction learning may decrease the quality of the tracking error signal, rather than increasing it.
Moreover, the conventional optical information reproducing apparatus cannot adjust the amplitude of the tracking error signal to a predetermined value, so that it cannot correct the variation in the overall gain in the tracking control system due to the variations in the characteristics of the optical head, the information recording medium, and the circuits, and the control performance may therefore be lowered.
The invention has been made to solve the problems described above, and its first object is to provide an optical information reproducing method and apparatus in which by just changing the combination of the input signals to the phase comparison means of the tracking error signal detecting circuit by means of the phase difference method, the phase adjustment amounts adjusted by the phase adjusting means for the respective output signals of the optical detectors can be set to the best values, without affecting the cost of the apparatus, and without decreasing the quality of the tracking error signal.
A second object of the invention is to provide an optical information reproducing method and apparatus which can cancel the electrical offset.
A third object of the invention is to provide a means for setting the amplitude of the reproduced tracking error signal to a predetermined level.
According to one aspect of the invention, there is provided an optical information reproducing apparatus for reading recorded information by having a light spot trace an information track consisting of a sequence of information pits by which information is recorded on an information medium, comprising:
a light source emitting a light beam;
photo-electric conversion means including first to fourth optical detectors;
an optical system for passing the light beam from the light source to the photo-electric conversion means via the information medium;
the first to fourth optical detectors converting the light beam into electrical signals,
the first to fourth optical detectors being situated in the far-field of the information pits in separate quadrants of an imaginary X-Y coordinate system, whose origin is disposed on an optical axis of the optical system and whose X-axis effectively extends in the track tangential direction and whose Y-axis effectively extends transversely to the track tangential direction,
the first and second optical detectors being disposed on one side of the first and second optical detectors being disposed on one side of the Y-axis,
the third optical detector being disposed on the other side of the Y-axis, and disposed diagonally with respect to the first optical detector, and
the fourth optical detector being disposed on the other side of the Y-axis, and disposed diagonally with respect to the second optical detector;
first to fourth phase adjusting means for individually adjusting the phase of each of the outputs from the first to fourth optical detectors;
phase difference detecting means for detecting the phase difference between the first sum signal obtained by adding the output of the first phase adjusting means and the output of the second phase adjusting means, and the second sum signal obtained by adding the output of the third phase adjusting means and the output of the fourth phase adjusting means, or the phase difference between the third sum signal obtained by adding the output of the first phase adjusting means and the output of the third phase adjusting means, and the fourth sum signal obtained by adding the output of the second phase adjusting means and the output of the fourth phase adjusting means;
offset correction learning means for adjusting the first to fourth phase adjusting means in accordance with the output of the phase difference between the first and the second sum signals from the phase difference detecting means, so as to eliminate the phase difference; and
tracking control means for obtaining the tracking error signal information from the output of the phase difference between the third and fourth sum signals from the phase difference detecting means.
The phase difference detecting means may comprise a switch means for switching between the phase difference between the first and second sum signals, or the phase difference between the third and fourth sum signals, in accordance with the output from the offset correction learning means.
The offset correction learning means may comprise:
offset adjusting means for adjusting the electrical offset superimposed with the output of the phase difference detecting means;
offset measuring means for measuring the output from the offset adjusting means, to determine the electrical offset;
reproduction level measuring means for measuring the output amplitude of the offset adjusting means; and
control means responsive to the output from the offset measuring means and the output from the reproduction level measuring means, for controlling the phase difference detecting means, the offset adjusting means, and the phase adjusting means.
The phase difference detecting means may be configured to detect, by a phase comparison means, the phase difference between the signals, and convert, by a low pass filter, the phase difference into a voltage, with the gain for conversion to the voltage being varied by variation of the gain of the low-pass filter.
The phase difference detecting means may be configured to detect, by phase comparison means, the phase difference between the two signals, and convert, by a charge pump and a low-pass filter, the phase difference into a voltage, with the gain for conversion to the voltage being varied by variation of the drive current to the charge pump or the gain of the low-pass filter.
According to another aspect of the invention, there is provided an optical information reproducing method for reading recorded information by having a light spot trace an information track consisting of a sequence of information pits by which information is recorded on an information medium,
the method being implemented by the use of an optical information reproducing apparatus for reading recorded information by having a light spot trace an information track consisting of a sequence of information pits by which information is recorded on an information medium, comprising:
light source emitting a light beam;
photo-electric conversion means including first to fourth optical detectors;
an optical system for passing the light beam from the light source to the photo-electric conversion means via the information medium;
the first to fourth optical detectors converting the light beam into electrical signals,
the first to fourth optical detectors being situated in the far-field of the information pits in separate quadrants of an imaginary X-Y coordinate system, whose origin is disposed on an optical axis of the optical system and whose X-axis effectively extends in the track tangential direction and whose Y-axis effectively extends transversely to the track tangential direction.
the first and second optical detectors being disposed on one side of the Y-axis,
the third optical detector being disposed on the other side of the Y-axis, and disposed diagonally with respect to the first optical detector, and
the fourth optical detector being disposed on the other side of the Y-axis, and disposed diagonally with respect to the second optical detector;
the method comprising steps of:
(a) conducting initial setting when the operation in an offset correction learning mode is started;
(b) selecting a first phase comparison mode in which a phase difference between a first sum signal obtained by adding a first phase adjustment output and a second phase adjustment output, and a second sum signal obtained by adding a third phase adjustment output and a fourth phase adjustment output, said first to fourth phase adjustment outputs being obtained by individually adjusting the phase of the output of each of the first to fourth optical detectors;
(c) driving the lens in a first one of the radially inward and radially downward directions of the information medium;
(d) detecting the phase difference in the operation in the first phase comparison mode, in a state in which the lens has been driven to the first one of the radially inward and radially outward directions of the information medium;
(e) individually adjusting the phase of the output of each of the first to fourth optical detectors so as to eliminate the phase difference detected at step (d);
(f) driving the lens in a second one of the radially inward and radially outward directions of the information medium;
(g) detecting the phase difference in the operation in the first phase comparison mode, in a state in which the lens has been driven to the second one of the radially inward and radially outward directions of the information medium;
(h) individually adjusting the phase of the output of each of the first to fourth optical detectors so as to eliminate the phase difference detected at the step (g); and
(i) adjusting the adjustment amount at the step (e), and the step (h).
The method may further comprise the steps of:
selecting a second phase comparison mode for detecting the phase difference between a third sum signal obtained by adding the first phase adjustment output and the third phase adjustment output, and a fourth sum signal obtained by adding the second phase adjustment output and the fourth phase adjustment output;
making an adjustment so that the reproduction level of the phase difference output detected in the second phase comparison mode is within a permissible range; and
making an adjustment so that the electrical offset superimposed on the phase difference output is within a permissible range.
The method may further comprise the steps of:
selecting the second phase comparison mode for detecting the phase difference between a third sum signal obtained by adding the first phase adjustment output and the third phase adjustment output, and a fourth sum signal obtained by adding the second phase adjustment output and the fourth phase adjustment output;
the step of selecting the second phase comparison mode being conducted after the step (e) and the step (g); and
adjusting the amplitude of the tracking error signal.
By setting the phase adjustment amounts of the first to fourth phase adjusting means so that the phase difference between the first sum signal and the second sum signal is eliminated, deterioration of the tracking error signal generating depending on the pit depth and the lens position is reduced, and the tracking error signal is obtained from the phase difference between the third sum signal and the fourth sum signal.
By switching the signals which are compared at the phase comparison means, the tracking error signal or the first offset which varies depending on the pit depth and the lens position, problematical in obtaining the tracking error signal in the phase difference method, can be detected directly.
By adding the function for correcting the gain variations in the tracking control system due to variations (due for example to manufacturing tolerances) in the characteristics of the circuits, the optical heads, or the like, and the electrical offset due to the circuits, causing deterioration of the tracking error signal, a tracking error signal with a high quality can be obtained.
According to the method described above, in order to unequivocally determine the phase adjustment amounts of the first to fourth adjusting means, while reducing the effects of the lens position, the lens is moved radially inward and radially outward, and the phase difference adjustment amounts for minimizing the phase difference between the first and second sums are determined.
Moreover, the electrical offset superimposed with the tracking error signal is canceled, and the amplitude variation of the tracking error signal due to variations (due for example to manufacturing tolerances) of the characteristics of the respective blocks used for implementing the invented method, and the reliability of the tracking control system is improved.
After canceling the first offset superimposed with the tracking error signal due to the pit depth and the lens position, the phase difference between the third and fourth sum signals is detected, and is converted into a voltage signal. In this way, the tracking error signal is obtained.
According to a further aspect of the invention, there is provided an offset removing circuit for an optical information reproducing apparatus for reading recorded information by having a light spot trace an information track consisting of a sequence of information pits by which information is recorded on an information medium, comprising:
a light source emitting a light beam;
photo-electric conversion means including first to fourth optical detectors;
an optical system for passing the light beam from the light source to the photo-electric conversion means via the information medium;
the first to fourth optical detectors converting the light beam into electrical signals,
the first to fourth optical detectors being situated in the far-field of the information pits in separate quadrants of an imaginary X-Y coordinate system, whose origin is disposed on the optical axis of the optical system and whose X-axis effectively extends in the track direction and whose Y-axis effectively extends transversely to the track direction,
the first and second optical detectors being disposed on one side of the Y-axis,
the third optical detector being disposed on the other side of the Y-axis, and disposed diagonally with respect to the first optical detector, and
the fourth optical detector being disposed on the other side of the Y-axis, and disposed diagonally with respect to the second optical detector;
the offset removing circuit comprising:
first to fourth phase adjusting means for individually adjusting the phase of the output of each of the first to fourth optical detectors;
phase difference detecting means for detecting the phase difference between the first sum signal obtained by adding the output of the first phase adjusting means and the output of the second phase adjusting means, and the second sum signal obtained by adding the output of the third phase adjusting means and the output of the fourth phase adjusting means, or the phase difference between the third sum signal obtained by adding the output of the first phase adjusting means and the output of the third phase adjusting means, and the fourth sum signal obtained by adding the output of the second phase adjusting means and the output of the fourth phase adjusting means; and
phase adjustment amount setting means for setting the phase adjustment amount of the first to fourth phase adjusting means so that the phase difference between the first sum signal and the second sum signal is made zero.
The offset removing circuit may further comprise an offset adjusting means for adjusting the electrical offset amount of an electrical circuit forming the optical information reproducing apparatus when the operation of the phase difference detecting means is halted.